It doesn't. There's nothing a "license" can do to prevent that. If I own the copyright to something, and I license it to party A under license X and party B under license Y, that's perfectly legal. This happens all the time in the commercial world where big software licenses are often written per-customer. If you make one of those licenses a blanket GPLv3 grant to the whole world, that's perfectly fine.

Where it gets sticky is the "ownership" issue. This only works if you're the sole copyright holder. If you've taken contributions from community members to your GPL project, you can't then relicense those to anyone else without approval, of course. This is the big reason behind the desire on the part of big projects for copyright assignment aggreements. Even the FSF does it so they can migrate software to GPLv4 when needed.

No, to find the answer, fall back to copyright law. If you own the copyright to some body of code in entirety, you can license it under as many licenses as you want. GPL for the community, proprietary trade-secret in exchange for money, and everything works fine.

But.... if you take a pull request from someone, either they have to sign over copyright to their patch, or else you have to involve them in every relicensing negotiation, because now they are a copyright holder, too, in the work.